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ACRYLIC (AND METHACRYLIC)
ACID POLYMERS

Introduction

Almost all acrylic polymers produced commercially contain acrylic acid and/or
methacrylic acid at some level, which gives them a special property. Content of
the acidic monomers ranges from

<5 wt% present in many widely used emul-

sion copolymers to100 wt% in water-soluble homopolymers. Between these ex-
tremes, by the inclusion of a wide variety of comonomers, the copolymers may
be water-soluble, soluble only in their neutralized forms, the so-called alkali-
soluble polymers, or swollen gels if cross-linkers are present. The less expensive
acrylic acid produced by a simpler process is now more commercially available
than methacrylic acid and dominates the polymer market especially in nonemul-
sion areas. This article focuses chiefly on water-soluble, alkali-soluble, and water-
swellable gels, based on homopolymers and copolymers since these are very im-
portant commercially and they contain significant levels of the acidic monomers.
Acrylic emulsion polymers have low acid content and are widely used in indus-
try and household paint formulations: they are included here in a selected limited
fashion because they have been widely studied and the open and patent literatures
are so voluminous.

Monomers

The

structures

of

propenoic

acid,

commonly

called

acrylic

acid,

and

2-methylpropenioc acid, commonly called methacrylic acid, are shown below:

Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

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Table 1. Physical Properties of Acrylic and Methacrylic Acids

Property

Acrylic acid

Methacrylic acid

Formula weight

72.06

86.10

Melting point,

◦

C

13.5

14.0

Boiling point, 101 kPa,

a

◦

C

141.0

159–163

Vapor pressure, 25

◦

C, kPa

a

0.57

0.13

Density, 25

◦

C, g/mL

1.045

1.015

Heat of vaporization, J/g

b

435

418

Heat of polymerization, kJ/g

b

1.08

0.657

Heat of polymerization, kJ

b

/mol

76.99

56.32

Heat capacity, 25

◦

C, J

b

/(g

·

◦

C)

2.1

2.1

Refractive index

η

D

1.4185

1.4288

Viscosity, 25

◦

C, mPa

·s (=cP)

1.25

1.3

Flash point, tag closed cup,

◦

C

50

67

Flash point, Cleveland open cup,

◦

C

68

77

Surface tension, 25

◦

C, mN/m (

=dyn/cm)

–

26.5

Solubility in water

Miscible

Miscible

Autoignition temperature,

◦

C

412

400

Dissociation constants (10

5

)

5.5

2.2

a

To convert kPa to mm Hg, multiply by 7.5.

b

To convert J to cal, divide by 4.184.

Acrylic acid and methacrylic acid are selective oxidation products of propy-

lene and isobutylene, respectively. The minor difference in structure between the
two acids, the

α-methyl substituent of methacrylic acid, results in minimal physi-

cal property differences as indicated in Table 1. However, this structural difference
distinctly affects polymerization and copolymerization kinetics and the character-
istics of the resulting polymers.

Acrylic acid was first reported in 1843 as a product of the air oxidation

of acrolein, which is obtained from the high temperature cracking of glycerol
(1) (Fig. 1). However, large-scale production of acrylic acid was not introduced
until the 1930s when industrial manufacturing processes were developed and
safe-handling procedures adopted for this very reactive monomer (2). Since then,
commercial growth has been phenomenal and, today, the production and use in

Fig. 1.

Cracking of glycerol to acrylic acid.

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81

polymerizations greatly predominates over methacrylic acid. This is due to its
lower cost and relative ease of preparation by the oxidation of inexpensive propy-
lene, and to a great extent to the global acceptance of superabsorbent acrylic
acid-based polymers in diapers and feminine hygiene products.

Frankland and Duma first reported methacrylic acid in 1865 (3). However,

like acrylic acid, its commercial development came many years later in 1933
(4). The development required pioneering chemistry on the monomer and its
derivatives by Dr. Otto Rohm published in his doctoral thesis in 1901 and its
evolution through the formation of the Rohm and Haas Co. in 1909 (5–8). Like
the acrylic acid, methacrylic acid and its vinyl derivatives are used mainly in the
preparation of polymers. There is little interest in small organic molecules based
on the monomers.

Properties.

The physical properties of the two acidic monomers are con-

trasted in Table 1. The major differences to note between the two acidic monomers
with regard to polymerization reactions and polymer properties are the higher
boiling point of the methacrylic acid and the higher heat of polymerization and
higher acidity of acrylic acid.

Both monomers are bifunctional organic compounds, with each having car-

boxylic acid and conjugated unsaturated double bond functionalities. The carboxyl
group undergoes all the reactions expected and readily forms salts and esters us-
ing conventional procedures. Caution must be exercised during these reactions
because the conjugated unsaturation present in the molecules renders them sus-
ceptible to highly exothermic and potentially explosive Michael addition reactions
and free-radical polymerizations under uncontrolled conditions. It is this high
reactivity of the monomeric double bond that allows rapid carefully controlled
polymerization to produce the expected result in polymerization reactions.

Manufacture of Acrylic Acid Monomer.

Many synthetic routes to

acrylic acid exist and are reported widely in the literature. However, the only
currently commercially viable process is based on the direct high temperature
gas-phase oxidation of propylene, as indicated schematically in Figure 2. This
economical process, based on the availability and low cost of propylene, is expected
to predominate commercially for many years. Other once practiced processes
based on acrylonitrile, propiolactone, and ethylene cyanohydrin are no longer of
importance (2).

The oxidation proceeds by the two-step catalytic process indicated above,

without the isolation of the intermediate acrolein. Acrylic acid, obtained in high
yield based on propylene, is isolated as an aqueous solution and purified by ex-
traction and distillation. The principal impurities in the commercial monomer,
termed glacial acrylic acid, that may affect subsequent polymerization reactions
include acetaldehyde, acrolein, furfural, benzaldehyde, and acrylic acid dimer, 3-
acryloxypropionic acid. Since these impurities are inhibitory in polymerization,

Fig. 2.

Acrylic acid by propylene oxidation.

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Fig. 3.

Processes for methacrylic acid and methyl methacrylate.

they must be removed to facilitate the production of ultrahigh molecular weight
polymers for use in superabsorbent manufacture, for example.

Manufacture of Methacrylic Acid Monomer.

The commercial manufac-

ture of methacrylates began in 1933 based on acetone cyanohydrin, which is still
the basis for all current production (4). This well-established technology is based
on the commercially inexpensive chemical starting compounds, acetone, hydro-
gen cyanide, and sulfuric acid. The process is represented by the schematic shown
in Figure 3. Acetone reacts with hydrogen cyanide to form acetone cyanohydrin,
which is readily converted into the methacrylamide sulfate salt in the presence
of anhydrous sulfuric acid. This intermediate salt may be directly converted into
either methacrylic acid or methyl methacrylate by reaction with water or anhy-
drous methanol, respectively. In commercial practice, the two steps are generally
carried out simultaneously with an aqueous methanol solution to produce an ester
rich mixture, since methyl methacrylate is the higher volume commercial product
for the production of Plexiglas (Rohm and Haas Co.).

Handling and Storage of Acrylic and Methacrylic Acids.

Acrylic

acid is available commercially at 98%

+ purity and contains a polymerization in-

hibitor such as hydroquinone (HQ), monomethylhydroquinone (MEHQ), or phe-
nothiazine. Methacrylic acid, on the other hand, is available at 99%

+ purity and

is stabilized with either HQ or MEHQ. It is not usually necessary to purify the
monomers to remove the inhibitors prior to polymerization, as the level of free-
radical initiators used readily overwhelms them. However, where ultrahigh molec-
ular weight polymers are required, for example in superabsorbent polymers, the
impurities must be removed. The corrosiveness of the monomers toward many
metals requires storage to be in stainless steel, glass, or aluminum or suitably
lined vessels. Any metals leached from storage vessels could potentially promote
dangerous runaway polymerizations in the presence of oxygen. Obviously, for

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the same reasons, polymerization reactions must be carried out in similarly con-
structed equipment.

Since both acids have relatively high freezing points, 13–14

◦

C, they must be

prevented from freezing in cold conditions. If freezing occurs, thawing is a haz-
ardous procedure since the distribution of inhibitor between solid and liquid is dif-
ficult to control, and runaway uncontrolled and potentially explosive polymeriza-
tion of the liquid phase is always a threat. To avoid this potential hazard, monomer
storage at temperatures in excess of their freezing points is recommended.

Health and Safety.

Both acids are moderately toxic compounds when in-

gested or absorbed through the skin. Severe internal burns result from swallowing.
Skin contact causes rapid tissue damage due to the highly corrosive nature of the
monomers. The vapor of acrylic acid is more irritating to the eyes than that of
methacrylic acid, but both may cause redness on prolonged exposure.

As with all industrial chemicals with similar characteristics, safe handling

with suitable protection gear is highly recommended; gloves, facemask, and cloth-
ing are all important. These are all listed in the Material Safety Data Sheets
(MSDS) that accompany the monomers and these sheets must be thoroughly read
and understood before working with either monomer.

Polymerization of Acrylic and Methacrylic Acids

Dependence on pH.

The rates of polymerization of both acidic monomers

in dilute aqueous solutions are pH-dependent. The rate of polymerization is high
at low pH for both monomers; it falls rapidly to a minimum at pH 6–7, and then
increases to a maximum at pH 10 for acrylic acid and 12 for methacrylic acid (9–
13). At high salt or monomer concentrations, the polymerization rate minimum
at the pH range 6–7 becomes less pronounced.

The explanation of the observed rate minimum at pH 6–7 is generally con-

sidered to be due to the slower rate of propagation for the anion than for the free
acid. However, as pH is increased, the rate of polymerization increases because
of the decreased rate of termination of the radical anions promoted by coulombic
repulsion (10). An alternative explanation of both the increase of polymerization
with pH and the leveling of the rate by added salts at pH 6–7 is postulated to be
due to the formation of terminal ion pair radicals with a propagation rate similar
to that of the free-acid monomers (9).

Copolymerization.

Acrylic and methacrylic acids readily copolymerize

free radically with many vinyl monomers. This versatility results from a com-
bination of their highly reactive double bonds and their miscibility with a wide
variety of water- and solvent-soluble monomers. Reactivity ratios derived from
copolymerizations with many monomers are tabulated in many books on polymer-
ization, for example in Wiley’s Polymer Handbook (14) (see also Wiley’s Database
of Polymer Properties). Q and e values are parameters that may be established
for a monomer based on a large number of reactivity ratios with other monomers.
These parameters are associated with interactions between the monomer and the
growing chain via resonance (Q) and polar effects (e).

As mentioned, the rate of copolymerization of acrylic and methacrylic acids

is dependent on pH effects and the resulting ionization as well as the comonomers.

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Table 2. Comonomer Effects on Acidic Polymers

Comonomer

Polymer

None

Water-soluble

Water-soluble

Water-soluble

Water-insoluble, low level

Water-soluble

Water-insoluble, mid level

Alkali-soluble emulsion

Water-insoluble, high level

Emulsion polymer

Polyvinyl cross-linker

Gel

This conflict makes it difficult to describe the acidic monomers by single Q and e
values derived from reactivity ratios. Neutralization changes both Q and e values.
Particularly, e values reverse from a positive value to a negative value on neu-
tralization (15). This is expected as e is a measure of electron-donating ability of
the monomeric double bond and neutralization may be equated to a change from
electron-withdrawing carboxylate group to an electron-donating carboxylate ion.

Alternative Synthesis of Acidic Polymers.

There are two approaches

to homo- and copolymers of acrylic and methacrylic acids. In addition to the con-
ventional use of acrylic acid and methacrylic acid monomers, the main theme of
this article, there exists the possibility of converting polymers of the derivatives
of these two monomers to acidic polymers. There would obviously have to be very
extenuating circumstances to take this route industrially because of cost penal-
ties. However, there are situations where there is a reason to do this. Availability
of monomers is a good example. Acrylonitrile was at one time more available than
acrylic acid in some parts of the world and simple hydrolysis of the polymer gave
poly(acrylic acid). Other potential routes exist from such homo- and copolymers of
acrylamide, acrylic and methacrylic esters, and acid chlorides. Although not fur-
ther discussed here, the reader is reminded that polymer synthesis with acrylic
monomers is very versatile and forethought is always necessary before plunging
ahead.

Polymerization Processes.

A variety of processes are used commercially

to homopolymerize and copolymerize acrylic acid and methacrylic acid. On the
basis of economics and environmental considerations, water is generally the pre-
ferred industrial solvent or polymerization medium. However, the choice of process
is usually dictated by the requirements of the polymer to be produced. As already
indicated, pH influences the rate of polymerization. Comonomers and molecular
weight of the polymer to be produced also have a profound effect on the type
of polymerization process that can be used and on the type of product obtained.
The contents of Table 2 indicate the change from water-soluble to alkali-soluble
emulsions and ultimately emulsion polymers is dependent on the comonomers in
copolymers of acrylic and methacrylic acids. This transition from water-soluble
polymer to emulsion polymer as the acidic monomer is decreased depends on the
hydrophobicity of the comonomer. Introduction of divinyl monomers causes tran-
sition to gel materials in all compositions. The gels may vary from highly swollen
to tightly bound copolymers, depending on the cross-linker level.

The effect of molecular weight is very pronounced with the water-soluble

polymers. The change of viscosity of aqueous solutions is shown schematically in
Figure 4. A precipitous increase in viscosity occurs at lower concentrations with

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ACRYLIC (AND METHACRYLIC) ACID POLYMERS

85

Weight percent solids

Point of rapid increase varies with MW

Viscosity

Fig. 4.

Water-soluble polymers: viscosity/molecular weight in water.

higher molecular weights, essentially making it economically impossible to make
polymer of greater than about 100,000 Da by aqueous solution polymerization.
This can be controlled to some extent by polymerizing the monomers at lower
concentration, but this becomes economically undesirable at some point, usually
around 30 wt% solids. Hence, new technology is borne. Generally speaking, low
molecular weight homopolymers, less than 100,000 Da, may be made in aqueous
solution; higher molecular weights require technologies such as suspension and
gel polymerization.

Copolymer synthesis suffers the same viscosity restrictions as the homopoly-

mers when the comonomers are sufficiently water-soluble to produce water-soluble
copolymers. When water-insoluble comonomers are used it is possible to resort to
cosolvents with water or surfactants in micellar polymerizations to effect solu-
bility to a limited degree, but this is only useful at low molecular weight and
is undesirable environmentally. Higher molecular weight copolymers are usually
made by emulsion, inverse emulsion, or suspension polymerization.

Polymer Characteristics

The polymer types considered here are primarily those that are soluble in water as
prepared, soluble after neutralization of emulsion polymerized copolymers, the so-
called alkali-soluble polymers, and cross-linked swellable gel polymers. Emulsion
polymers are discussed in a limited fashion, as acrylic and methacrylic acids are
used ubiquitously at low levels in almost all acrylic emulsion polymerizations to
contribute some special characteristic. For example, the incorporation of acidic
monomers contributes to such properties as adhesion, wettability, and emulsion
polymer stability.

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Emulsion Polymers.

Applications.

Acrylic emulsion polymers containing insufficient acrylic or

methacrylic acids to solubilize on neutralization are used in a myriad of appli-
cations including, coatings, floor polish, inks, dispersants, adhesives, and caulks.
These are almost all thin film applications and the acid contributes hydrophilic-
ity, stability, dispersancy, etc. The polymers are balanced by backbone hardness,
hydrophobicity/hydrophilicity balance, functionality, all moderated by choice of
comonomers and by molecular weight and morphology. Some examples from
the literature of the use of emulsion copolymers containing low levels of acidic
monomers include autodeposition coatings (16), paper saturants (17), wood coat-
ings (18), corrosion-resistant coatings (19), printing ink binders with core-shell
morphology (20), monodispersed particles for liquid toners (21), oil-repellent fin-
ishes based on fluoromonomers (22), dilatant dispersants for antistatic coatings
(23), water-resistant coatings (24), water-dispersible polymers for building mate-
rials (25), surfactant-free emulsions for nonwovens (26), pressure-sensitive adhe-
sives with good mechanical stability (27), water-resistant cosmetics (28), anticor-
rosion coatings for ferrous metals (29), hair styling shampoos (30), redispersible
film-forming coatings (31), core-shell graft polymers for cement modifiers (32),
coatings and inks (33), wood coatings (34), acrylic polyester hybrids (35), redis-
persible powders for cement (36), hair fixatives (37), redispersible core-shell poly-
mers (38), core-shell coatings (39–42), coatings (43), pressure-sensitive adhesives
(44), and emulsion paints (45).

Synthesis.

The synthesis of emulsions is widely described in the literature,

both patent and open. Ingredients are surfactants for setting particle size and
stabilization of the emulsion, and initiators for polymerization. Leading reference
books are by Blackley; although somewhat dated it is still an excellent book and
another is Gilbert’s recent publication (46). Other useful synthesis references in-
clude some specific interests, pressure-sensitive adhesives (47), styrene emulsion
polymerization using alkali-soluble resins as emulsifiers (48), manufacture and
use of curable emulsions (49), preparation of concentrated emulsions (50), mul-
tistage polymer preparation (51), core-shell vinyl acrylic emulsions (52), acrylic
grafted to polyester emulsions (53), aqueous polymer dispersions (54), process for
polyacrylate dispersions (55), sterically stabilized acrylic graft copolymers (56),
polyacrylate dispersions without colloid protection (57) and with colloid protection
(58), low VOC emulsions (59), design of latex polymers (60), random copolymers
as emulsifiers (61), multihollow latex particles (62), modeling of acid distribution
in lattices (63,64), modeling of microparticles and experimental verification (65),
and effect of pH on properties of emulsion polymers (66). Since so many applica-
tions of acrylic emulsions are dependent interfacial science, film formation is very
important and has received much interest (67–70).

Water-Soluble Polymers.

Synthesis.

Water-soluble polymers are prepared by conventional free-

radical polymerization methods with molecular weight control through the level of
initiator and the use of chain transfer agents. Generally, molecular weights of the
polymers range from a few thousand to several hundred thousand, which is useful
in many applications from dispersants to rheology modifiers to thickeners to floc-
culants. The lower molecular weights from 1000 to about 30,000 have been most
widely worked on. Typical initiators fall into two classes, the thermally activated

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87

Fig. 5.

Chain transfer in free-radical polymerization.

and the redox types. The thermally activated are persulfates, perphosphates, azos,
hydroperoxides, etc, which are used in temperature ranges of 50–100

◦

C. The redox

initiators are generally designed for use at lower temperatures and function by
using stoichiometric amount of reducing and oxidizing agents in the presence of
a metal catalyst. The metal decomposes the peroxy compound to produce radicals
for initiating polymerization and is converted to a higher oxidation state in the
process; the reducing agent cycles the metal back to the lower oxidation state and
the process is repeated.

Molecular weight control by using chain transfer agents (Fig. 5) results in

the incorporation of the chain transfer agent at least on the polymer terminus.

RX is a general term for a chain transfer agent. The liberated R

· radical

then initiates another polymer chain and X, usually hydrogen from chain trans-
fer agents, terminates the current chain. Phosphite chain transfer agents are
exemplified in the aqueous preparation of low molecular weight acrylic homo-
and copolymers (71,72). A similar utilization was patented by Coatex S. A. (73).
Other chain transfer agents that have been used include alcohols such as iso-
propylalcohol (74), aminothiols (74), mercaptoethanol (75), mercaptoethanol (76),
hypophosphorous acid (77), and copper salts that are extremely effective (78–82).

Other interesting polymerizations include the use of metal-activated hy-

drogen peroxide to deliver low molecular weight polymers (83,84), continuous
polymerization of water-soluble monomers in extruders (85), dry polymerization
of acrylic acid in super critical carbon dioxide (86,87) and on a powder bed (88),
and the use of sodium nitrate mediated aqueous polymerization to allow high
solids (89).

Transamidation allows the preparation of unique low molecular weight,

water-soluble polymers from acrylic acid–acrylamide copolymers. The advantage
is that functional amines such as sulfonic acid salts can be incorporated into pre-
formed polymer to create new functional polymers without the need to synthesize
new monomers (90–92).

Applications.

Water-soluble homopolymers of acrylic and methacrylic acids

and their copolymers are widely used in a myriad of applications. Poly(acrylic
acid) and copolymers with maleic acid and other monomers are used in large
quantities in solid detergents and dishwashing powders (80,93–107). Only mi-
nor modifications to composition and/or molecular weight to alter the balance
of surface interactions allow use in many different applications. Changing hy-
drophile/hydrophobe ratio, charge density, introduction of other charged species,
etc, all expand the application range. Scale formation in aqueous systems may be
prevented by the presence of sulfonate groups (108–110) and allyloxy function-
ality (111,112). Cement additives used to reduce water requirements are based
on acrylic acid copolymers with poly(ethylene oxide) acrylate monomers (113–
116). Copolymers with acrylic alkyl esters are used in textile sizing agents (117).

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Plant productivity in hydroponic media is reportedly enhanced by the presence
of poly(acrylic acid) (118). Dispersing characteristics are imparted by copolymer-
izing a range of monomers and again they are used widely, for example, in inks
(119), for inhibiting barium scale in seawater (120), ceramics (121,122), to disperse
acrylic emulsion polymers (123), to disperse dyes (124). Copolymers are also used
in oil drilling and recovery (125–129), in corrosion control (130–136), in paper for
wet and dry strength (137), boiler water treatment (138–141), specialty cleansers
(142–146), eliminating odor from animal excrement (147), reverse osmosis for
scale inhibition (148), water clarification (149), soil conditioners (150), and seed
coatings (151).

Environmental Issues of Water-Soluble Polymers.

Because the water-

soluble polymers based on acrylic and methacrylic acids are generally disposed
off in the environment, there have been studies to determine their effect on
the environment and to monitor their build. Acrylic acid polymers are not
biodegradable except at the oligomer molecular weight, that is about 3 to 10 units
(152). Elegant monitoring of low concentrations is available by two techniques,
immunoassay (153) developed at Rohm and Haas, and fluorescence tagging
(154). Tagging is useful both for environmental monitoring and for concentration
control in applications.

Alkali-Soluble Polymers.

Alkali-soluble polymers are generally emul-

sion polymers that have insufficient acid content to solubilize in water in the ab-
sence of neutralization. In addition to emulsion polymerization, polymerization in
partial or complete organic solvents, as suspensions, and inverse emulsions are all
reported. However, emulsion polymerization represents the most convenient route
to these polymers, with suspension and inverse emulsions being very limited.

Alkali-soluble emulsion polymers were originally developed as thickeners

based on copolymers of acrylic acid and methacrylic acid with simple acrylic and
methacrylic esters, as exemplified by their preparation in patents to S.C. Johnson
(155), BASF (156), and Goodyear (157). However, in the last two decades advanced
hydrophobically modified alkali-soluble polymers have emerged as thickeners and
rheology modifiers which function by association of their hydrophobic functionali-
ties with themselves or other convenient centers in a formulation containing these
polymers. Figure 6 represents this association schematically.

Both types of alkali soluble thickeners generally contain methacrylic acid

rather than acrylic acid since this monomer is more readily incorporated into

Fig. 6.

Association of hydrophobic alkali-soluble polymers.

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ACRYLIC (AND METHACRYLIC) ACID POLYMERS

89

emulsion polymers, as it is more hydrophobic than acrylic acid and less likely to
homopolymerize in the aqueous continuous phase. Patents to Dow (158), DeSoto
(159), Union Carbide (160), and Rohm and Haas (161) represent the synthesis of
hydrophobically modified alkali-soluble emulsion polymers. The hydrophobe may
be attached to the acrylic polymer backbone by an ester or urethane linkage (162).
The properties of hydrophobic associative thickeners and conventional thickeners
in paint-making formulations have been compared (163). Several papers have
appeared on the interactions of similar polymers in paint formulations (164–167)
and on general hydrophobic interactions in water (168,169).

There are other less widely used syntheses of associative polymers including

acid monomer grafting to poly(ethylene oxide) (170,171), inverse emulsion poly-
merization (172–174), suspension polymerization (175), micellar polymerization
(176,177), and solid-phase extrusion polymerization (178).

Applications.

In addition to latex paint applications already mentioned,

applications include paper (179), oil spill clean-up (180), flocculants and mineral
dewatering (181), emulsion polymerization stabilizers (182), and in mixed asso-
ciative thickeners in paints (183).

Gel Polymers.

Gel polymers based on the acidic monomers are high molec-

ular weight (co)polymers that are cross-linked such that solubilization in water
is prevented. The levels of swelling or gel formation are controlled by degree of
cross-linking of the polymer. The major outlet for this chemistry is in the develop-
ment of superabsorbent polymers now ubiquitous in diapers and feminine hygiene
products. They have completely revolutionized a way of life for raising infants and
caring for incontinent elderly. Hence, it is not surprising that most of the published
information is patent literature that relates to manufacturing processes that have
been or continue to be used for the preparation of superabsorbents. Some of these
processes are referenced herein, but the patent literature is so voluminous that
not all are included here.

Synthesis.

Gel polymerization of acrylic acid with a polyvinyl cross-linking

monomers in water is by far the most prevalent commercial process. Examples
of cross-linkers used are triallylamine (184), N,N



-methylenebisacrylamide (185),

tetraallyloxyethane (186,187), trimethylolpropane triacrylate (188), ethylene di-
amines (189), trimethylol triacrylate and diacrylate in combination (190), ethy-
lene glycol diglycidyl ether (191), trimethylolpropane tris(3-aziridinylpropionate)
(192), and poly(ethylene oxides) diacrylate (193). In an extension of cross-linking
one process by Sanyo uses inefficient graft polymerization of acrylic acid onto
starch as a source of the cross-linking required to introduce control over gel
swelling (194).

Applications.

Copolymers of acidic monomers and other water-soluble

monomers obtained by gel polymerization for special effects, generally in medical
applications, include hydroxyethyl methacrylate (195), hydroxypropyl methacry-
late and acrylamide (196), and vinyl saccharides (197).

There has been a move from conventional granular gel materials to foamed

(198) and flat plate (199) type superabsorbents based on poly(acrylic acid). In
addition to superabsorbents for personal hygiene, the materials have wide rang-
ing applications in many areas from biomedical to cosmetics. Some of these are
petroleum recovery and sealing of sewer pipes when cross-linked with metals
such as iron salts (200); flocculants in water purification (201); copolymers with

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acrylamide in textile printing pastes (202); cosmetic gelling agents (203); surgical
dressings (204); reversible gels (205); subterranean fault stabilization (206), and
controlled release (207).

Block and Graft Polymers

Graft

Synthesis.

Free-radical-promoted

grafting

of

acrylic

and

methacrylic acids and other water-soluble comonomers onto synthetic and
natural substrates gives unique polymeric structures quite different from the
random copolymers obtained in conventional copolymerization. These block and
graft polymers bring a unique and different structure/property balance with
applications in many areas.

Applications.

Poly(ethylene oxides) are readily grafted with acrylic acid to

give partially biodegradable detergent polymers (208–210). Grafts onto other sub-
strates include poly(vinyl alcohol) (211), water-soluble maleic anhydride copoly-
mers (212), polycaprolactone (213), and onto emulsion polymers of acrylates and
methacrylates (214).

Natural substrates are often starch or cellulose derivatives. Grafts for deter-

gent applications (215,216), thickening dispersions (217), leather tanning (218),
water treatment (219), oil recovery (220), water-absorbents (221), and water-
absorbents in fibers and sheets (222). More fundamental studies on grafting to
polysaccharides have been published (223).

Block Synthesis.

Water-soluble block copolymers are formed from

the copolymerization of macromonomers of methacrylates with acrylic and
methacrylic acid monomers and their solution properties compared with random
copolymers of similar composition (224). Diblock and triblock copolymers may
be prepared by a number of techniques and are also used on ink-jet inks (225)
and scale inhibition in water boilers (226), respectively. Associative properties of
block polymers to form micellar structures are well established (227,228). Triblock
polyampholyte polymers are also known (229).

Inverse Emulsion Polymerization

Inverse emulsion polymerization is a term used for water in oil (monomer) disper-
sion polymerization as opposed to conventional emulsion polymerization where
the monomer and polymer are largely insoluble and dispersed in water. The ad-
vantage is that high molecular weight polymers and copolymers of water-soluble
monomers may be prepared without the attendant viscosity build mentioned ear-
lier. The process chemistry is well established as indicated in the patent literature
(230–233) but not widely used and likely to be less so as solvents are removed from
processing. Spray drying is often used for polymer isolation (234).

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G

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